Show original headers bug in 7.1.0

Hi,
after upgrading to ZCS 7.1.0 users cannot see original raw headers of mail. When they click on "Show Original" popup window displays, asking for application to open with.
Popup window reveals Type: application/octet-stream

BUT this problem ocures only for some of messages in some of folders, let's say for about 1% of all messages. There is no any obvious rule, some text messages have problems, and some HTML also, with or without attachments.

Seems like message store or some index is broken.
Any idea how to fix this?

I don't see that behaviour. Have you tried re-indexing the account? Sounds like a browser problem to me, have you tried a different browser or another PC or a clean profile for the browser? Which browser are you using?

one paragraph obviously slipped out of the screen, since I was typing within interruptions
Yes, I have tried on 3 different computers, on IE and Firefox. Problem is present only on Firefox and only on some of the messages. I never faced this problem before, so I assume it was absent.